Process for producing multivalent metal pyrophosphates



United States Patent M t 3,401,012 PROCESS FOR PRODUCING MULTIVALENTMETAL PYRQPHOSPHATES George D. Nelson, St. Louis, Mo., assignor toMonsanto Company, St. Louis, Mo., a corporation of Delaware No Drawing.Filed Dec. 27, 1966, Ser. No. 604,599 11 Claims. (Cl. 23105) Thisinvention relates to the production of multivalent metal pyrophosphates.More particularly it relates to a process for the production of highpurity multivalent metal pyrophosphates using solid pyrophosphoric acidas a raw material.

Multivalent metal pyrophosphates have generally heretofore been preparedfrom the thermal dehydration of the appropriate oithoph-osphates.Multivalent metal orthophosphates, in many instances, are not readilyavailable. It is believed that a process which would allow theproduction of various multivalent metal pyrophosphates directly frompyrophosphoric acid and a more readily available multivalent metalsource would be an advancement in the art.

-In accordance with this invention, it has been found that high puritymultivalent metal pyrophosphate can be produced by reactingpyrophosphoric acid and a multivalent metal source in an aqueous mediumunder controlled conditions of temperature and pH to thereby form amultivalent metal pyrophosphate precipitate. More particularly, it hasbeen found that multivalent metal pyrophosphates can be produced byreacting pyrophosphoric acid and a multivalent metal source in anaqueous solution having a water content of at least about 85% by weightof the reaction medium and at temperatures of from about 17 C. to about48 C., and at a pH from about 1.5 to about 7; and that an easy-tofilter,high purity multivalent metal pyrophosphate precipitates from thereaction mixture. It is to be noted as used herein multivalent metalsmeans iron, cobalt, nickel, zinc, aluminum, tin, lead and copper.

Solid pyrophosphoric acid, H4P207, is a specific chemical compoundhaving a P 0 content of 79.76% and is not to be confused with the syrupyliquid having 21 P 0 content of from 78% to about 82% which is sometimesknown as liquid pyrophosphoric acid." The beforementioned liquid is amixture of pyro, ortho, and polyphosphoric acids and is not suitable inthe practice of this invention. Solid pyrophosphoric acid is known toexist in at least two forms-Form I crystalline material which melts at54 C. and Form 11 crystalline material which melts at about 71 C. Whenheated to its melting temperature, solid pyrophosphoric acid undergoes arearrangement to produce a mixture of pyro, ortho, and higherpolyphosphoric acids. Although any form can be used in the practice ofthis invention, Form II is preferred since it is more thermally stable.Pyrophosphoric acid in an aqueous solution also hydrolyzes to formorthophosphoric acid. Such hydrolysis is more rapid at relatively hightemperatures, that is, at temperatures above 50 C., therefore,temperatures below about 50 C. are to be used in the practice of thisinvention.

The other reactant used in the practice of this invention is awater-soluble multivalent metal source, that is, a multivalent metalsource which is soluble to the extent of at least about 5% by weight inwater at 25 C. The multivalent metal sources can include the salts ofinorganic acids as well as the multivalent metal bases such as themultivalent metal oxides and hydroxides. When the multivalent metalsalt-s are used, it is preferred to use salts of the inorganic acidssuch as the multivalent metal chlorides, carbonates, sulfates andnitrates and the multivalent metal salts of the lower al-kyl carboxylicacids (con- 3,401,012 Patented Sept. 10, 1968 taining from one to 4carbon atoms). Illustrative of the foregoing salts that can be used areferric chloride, ferric sulfate, zinc carbonate, cupric carbonate,cupric sulfate, lead acetate, lead sulfate, nickel sulfate, stannouschloride, nickel carbonate, cobalt nitrate, cobalt sulfate, ironformate, lead formate and Zinc butyrate.

In most instances, it is preferred to use the more common and relativelyless expensive inorganic multivalent metal salts such as the chlorides,carbonates, and nitrates or the multivalent oxides and hydroxides, suchas Zinc hydroxide, aluminum hydroxide, zinc oxide and ferric oxide.

In the process of this invention it is preferred to prepare a solutionof pyrophosphoric acid by adding solid pyrophosphoric acid to water andmaintaining the temperature of the resulfing solution at temperaturesbelow 50 C. and preferably from about 0 to about 10 C. It has been foundthat when solutions prepared in the foregoing manner are used, thereaction conditions can be controlled much easier than if solidmaterials or more concentrated solutions are used. It is believed thatin this manner both pH and the concentrations that are preferred can bemaintained in a more suitable manner. However, solutions containing fromabout 5 to about 15% pyrophosphoric acid can be used if desired.

The pyrophosphoric acid can be added to an aqueous solution or slurry ofa multivalent metal source with the water concentration of the resultingmedium at from about 84% to about by weight. If preferred, however, thetwo solutions can be added simultaneously to water at reactiontemperatures, pHs and concentrations. Furthermore, if desired, themultivalent metal source can be added to the pyrophosphoric acid,however, pH control is more difiicult to control at the: preferredrange, that is, from about 3 to about 5 at early stages of addition ofthe multivalent metal source because the pH of the pyroph-osphoric acidsolution is below about 2.

In most instances, it is necessary to control the pH of the reactionmedium by the addition of a basic material, such as ammonium hydroxide,sodium hydroxide or by a basic salt such as sodium carbonate, potassiumcarbonate and the like. The pH is preferably controlled between about 3and about 6 during the reaction for most of the multivalentpyrophosphates. In some instances, however, it is necessary to adjustthe pH to a value above 6 in order to obtain satisfactory precipitationfrom a solution. In most instances, it is necessary to have a pH valueabove about 3 at the completion of the reaction for the pyrophosphatesto precipitate readily from the solution. At pH values much below about1.5, orthophosphates tend to form and at pH values above about 7, mixedsalts containing various other cations tend to be formed therebycontaminating the product.

The temperature of the reaction medium should be controlled attemperatures below :about 50 C. to avoid excessive hydrolysis of thepyrophosphoric acid. Preferably the temperature of the reaction mediumshould be from about 17 C. to about 48 C. Although temperatures below 17C., that is as low as about 5 C. can be used, however, it is necessaryin most instances to provide some means of cooling such as a brinesystem to obtain these lower temperatures, therefore, it is since theselower temperatures do not achieve any additional beneficial resultsnormally they will not be used.

The water concentration in the reaction medium should be at least about84% by weight of the total reaction medium to enable adequate control ofthe process. Although there is no essential maximum water concentrationin order to achieve the benefits of this invention, it is generallypreferred to use a water concentration below about 95% in order to avoidproblems relating to disposal of an excessive amount of water. Although,if desired, water concentrations greater than 95% can be used (such as97 99%) and the water can be recovered and recycled. Waterconcentrations of less than about 84% can result in the contamination bycompounds which Would be water soluble at the higher waterconcentrations, therefore, are to be avoided. Thus, in most instances,it is preferred to maintain a Water concentration in the reaction mediumof from about 87% to about 95%.

The multivalent metal pyrophosphates that are produced by the process ofthis invention have extremely high purities. The main reason for thehigh purities of these materials is because of the relatively insolublenature of the multivalent metal pyrophosphates as compared to othersalts which may be formed in the reaction medium. This process,therefore, enables the production of salts having a multivalent metalpyrophosphate content of at least about 90% by weight. These high puritycompounds can be produced even if the pyrophosphoric acid has an assaywhich is relatively low or even if alkali metal ions or ammonium ionsare present, since the other salts which can be formed in the reactionmedium are soluble within the concentrations of water that are present;therefore, they will not crystallize from the reaction medium to causecontamination of the pyrophosphates unless the pH or temperature isallowed to exceed the range as specified herein.

The yields that are achieved in the practice of this invention arerelatively high, that is, above about 90%, based upon the amount ofmultivalent pyrophosphate that is produced from the pyrophosphoric :acidreactant.

Recovery of the pyrophosphates is achieved by filtration, centrifugationand the like. It is generally preferred to separate the multivalentmetal pyrophosphates from the reaction medium by filtration, followed bya Water solution wash to remove any of the materials which are containedin the aqueous portion of the cake. This can be done with water whichwill remove any of the trace amounts of materials which are present inthe cake. Similarly a centrifuge with a wash cycle provides an adequatemeans of recovery of the multivalent pyrophosphates produced by theprocess of this invention. In some instances, if the presence of theother salts which are present in the reaction medium would not bedetrimental, it is possible to separate the multivalent pyrophosphatesby allowing the multivalent metal pyrophosphates to settle from thereaction medium and then decanting the liquid portion from the solidpyrophosphates. In these instances, however, it must be noted that theaqueous material which is contained in the solids contains othercompounds that are soluble in the water, therefore, lower puritypyrophosphates are recovered in this manner. However, the solidpyrophosphate can be subjected to a water wash which will dissolve thesecompounds thereby yielding a high purity multivalent metalpyrophosphate.

The tin pyrophosphate is useful as a butter in certain I dentrificecompositions. Pyrophosphates such as the cobalt, copper, nickel and ironpyrophosphate are generally useful as catalysts in oxidation reactionsin gasoline purification. Moreover, many of the pyrophosphates producedby the process of this invention are useful in electrical plating baths.

To more specifically illustrate the process of this invention, thefollowing non-limiting specific examples are presented. All parts,proportions and presentations are by weight unless otherwise indicated.

EXAMPLE I About 358 parts of zinc oxide are slurried in about 4,000parts of water. About 356 parts of crystalline pyrophosphoric acid(79.6% P are dissolved in about 3,000 parts of water at 0 to C. Theaqueous solution of pyrophosphoric .acid prepared above is fed into astirred vessel containing the zinc oxide slurry at about C. and at arate of to parts per minute. The temperature of the reaction medium isheld at from about 23 C. to about 38 C. A pH of from about 3.0 to about3.7 is maintained throughout the reaction by the addition of a 2%solution of sodium hydroxide. Stirring is discontinued and about 5,300parts of liquid are then decanted from a solid precipitate. Theprecipitate is washed with water on a filter and air dried. A ZnOcontent of the precipitate of about 40.0% is determined by the ammoniumorthophosphatepyrophosphate gravimetric method as described in Willardand Furman, Elementary Quantitative Analysis, 3rd ed., D. Van NostrandCompany, New York (1940).

A P 0 content of about 36.6% of a sample of the product is determined bythe method of Van Wazer et al. described in Analytical Chemistry, 26,1755 (1954), after ion exchange separation of the metal ions by acationic exchange resin.

Ignition loss measurements determines the molecular bound water contentof a sample of the Product to be 23.2% H O.

The following analyses of P species present as ortho, end and middlegroups are obtained by the method described in Analytical Chemistry, 26,1755, supra.

Percent of phosphorus species Ortho Ends 99.1 Middles 0.0

EXAMPLE ll About 178 parts of crystalline pyrophosphoric acid aredissolved in about 3200 parts of water at a temperature of from about 0to 10 C. The temperature of this aqueous solution of pyrophosphoric acidis maintained at below 10 C. until it is added at the rate of aboutparts per minute to a stirred reaction vessel equipped with coolingcoils containing about 2000 parts of water at about 40 C. About 2600parts of cupric carbonate and about 3000 parts of water at a temperatureof about 25 C. are added simultaneously with the pyrophosphoric acidsolution. The temperature of the reaction medium while the materials arebeing added is maintained at about 25-33 C. by circulating brine atabout 15 C. through the cooling coils. The pH is maintained at about 2.4to 3.3 by the addition of ammonium hydroxide. After all of the acid andcupric carbonate solution are added, the pH is adjusted to about 5.0 bythe addition of a 10% solution of sodium hydroxide.

A granular precipitate is formed which is easily separated from theliquid solution phase by filtration.

Using the electrolytic analytical method as described in Kolthoff andSandell, Textbook of Quantitative Analysis, rev. ed. (1946), McMillanCompany, New York, for determining the CuO content and the previousmethods described in Example I for the remaining analyses the followinganalyses a-re obtained on samples of the granular material.

Percent CuO 43.8 P 0 38.2 P as ortho 0.0 P as ends 94.0 P as middles 6.0Loss on ignition 19.7

From these analyses it is apparent that the material produced isCU2P2O74H2O.

EXAMPLE III About 314 parts of crystalline pyrophosp'horic acid aredissolved in about 4000 parts of water. The temperature of this aqueoussolution of pyrophosphoric acid is maintained at below 10 C. until it isadded at the rate of about 50 parts per minute to a stirred reactionvessel equipped with cooling coils containing about 454 parts of ferricoxide and about 4,000 parts of water at a temperature of about 25 C. Thetemperature of the reaction medium while the pyrophosphoric acid isbeing added is maintained at about 23-24 C. by circulating brine atabout 15 C; through. the cooling coils and by agitating the reactionmedium. The pH of the reaction medium is maintained at from about 2.3 toabout 3.3 by the addition of a sodium hydroxide solution. After all ofthe acid is added, the pH is adjusted to about 6.0 by adding ammoniumhydroxide.

A granular precipitate is formed and about one hour after all of thepyrophosphoric acid solution has been added, the agitation is stoppedand the granular material is separated from the liquid solution phase byfiltration.

Using the KMnO volumetric method as described in Willard-and Furman,Elementary Quantitative Analysis, 3rd ed. (1940), D. Van NostrandCompany, New York, the Fe O content of the material is determined. Otheranalytical determinations are made in accordance with the techniquesdescribed in Example I. From these analyses, the material produced isdetermined to be Fe (P O -20H O.

EXAMPLE IV About 178 parts of crystalline pyrophosphoric acid aredissolved in about 3,000 parts of water at a temperature of about 10 C.The temperature of this aqueous solution of pyrophosphoric acid ismaintained at below 10 C. until-it is added at the rate of about 100parts per minute to a stirred reaction vessel equipped with coolingcoils containing about 2,000 parts of Water. A solution prepared byadding about 450 parts of stannous chloride dihydrate to about 5,000parts of Water at a temperature of 25 C. is added simultaneously withthe acid to the water in the react-ion vessel. The temperature of thereaction medium while the pyrophosphoric acid and the stannous chloridesolution are being added, is maintained at about 21 C. by circulatingbrine at about C. through the cooling coils and by agitation. The pH ofthe reaction medium is maintained at about 4.5 by the addition of sodiumhydroxide, until all of each solution is added. About 10 minutes afterthe solutions are added, the reaction medium is adjusted to a pH ofabout 7.0, with a 10% solution of ammonium hydroxide. A granularprecipitate is formed and the agitation is discontinued. The granularmaterial is separated from the liquid solution phase by filtration.

Using the analytical techniques as in Example I, the material isdetermined to be essentially anhydrous and has the formula, SIIgPzOq.

EXAMPLE V About 106 parts of crystalline pyrophosphoric acid aredissolved in about 2,000 parts of water at a temperature of from about 0to 10 C. The temperature of this aqueous solution of pyrophosphoric acidis maintained at below 10 C. until it is added at the rate of about 100parts per minute to a stirred reaction vessel equipped with cool ingcoils containing about 454 parts of lead acetate and about 4,000 partsof water at a temperature of about 25 C. The temperature of the reactionmedium while the pyrophosphoric acid is being added, is maintained atabout 25-35 C. by circulating brine at about 15 C. through the coolingcoils and by agitation. A pH of about 1.7 to about 4.2 is maintainedduring the addition of the acid by the addition of the 10% solution ofsodium hydroxide. -In about hour, after all of the pyrophosphoric acidis added, the agitation is discontinued and a granular materialseparates from the liquid phase.

Using the analytical techniques as in Example I, the material isdetermined to be essentially anhydrous and has the formula Pb2'P207. AnX-ray diffraction pattern on a sample of the material is identical tothe Pb P O prepared by thermal dehydration and reported at J. Am. Ceram,Soc. 43, 452 (1960).

6 EXAMPLE VI About 530 parts of crystalline pyrophosphoric acid aredissolved in about 4,000 parts of Water at a temperature of 10 C. Thetemperature of this aqueous solution of pyrophopshoric acid ismaintained at below 15 C. until it is added at the rate of about partsper minute to a stirred reaction vessel equipped with cooling coilscontaining about 340 parts of aluminum hydroxide and about 5,000 partsof Water at a temperature of about 25 C. The temperature of the reactionmedium while the pyrophosphoric acid is being added is maintained atabout 33-48 C. by circulating water through the cooling coils and byagitation. The pH of the reaction medium is maintained at about 2.2 toabout 2.4 by the addition of minor amounts of a 10% solution of ammoniumhydroxide until all of the pyrophosphoric acid is added. Additionalammonium hydroxide is then added to adjust the pH to about 6.0.,

A granular material is formed and the agitation is discontinued and thematerial is separated from the aqueous solution phase by decanting offthe aqueous solution phase.

Using the MP0; gravimetric method for determining A1 0 and by using theother methods as described in Example I, the following analyses areobtained on samples of the crystalline material.

From these analyses it is apparent that the material is Al (P O -20H O.Since no X-ray diffraction pattern could be obtained, the material isbelieved to be largely amorphous.

EXAMPLE VII About 178 parts of crystalline pyrophosphoric acid aredissolved in about 2,000 parts of water at a temperature of about 5 C.The temperature of this aqueous solution of pyrophosphoric acid ismaintained at below 10 C. until it is added at the rate of about 10 0parts per minute to a stirred reaction vessel equipped with coolingcoils containing about 2,000 parts of water at above 25 C. About 290parts of nickel carbonate are added to about 2,000 parts of water at atemperature of about 25 C. and the solution is fed simultaneously withthe pyrophosphoric acid to the water in the reaction vessel. Thetemperature of the reaction medium while the solutions are being addedis maintained at about 40 C. by circulating water through the coolingcoils and by agitating the reaction mixture. The pH is maintained atabout 3.5 to about 4.3 with the addition of ammonium hydroxide. Afterall of the solutions are added, the pH is raised to about 6.0 withsodium hydroxide.

A granular material is formed and is separated from the liquid solutionphase by filtration.

Using the analytical techniques as in Example II, the following analysesare obtained on samples of the crystalline material.

From these analyses it is apparent that the material is NigPgOq SHZO.

EXAMPLE VIII About 178 parts of crystalline pyrophosphoric acid aredissolved in about 3,000 parts of water at a temperature of about C. Thetemperature of this aqueous solution of pyrophosphoric acid ismaintained at below 10 C. until it is added at the rate of about 100parts per minute to a reaction vessel equipped with cooling coils and astirrer and which contains about 562 parts of cobalt sulfate and about4,500 parts of water at a temperature of about 45 C. The temperature ofthe reaction medium while the pyrophosphoric' acid is being added ismaintained at about 40 C. by circulating brine at about C. through thecooling coils and by maintaining agitation of the reaction medium. ThepH is held at about 4.0 by the addition of ammonium hydroxide until allof the pyrophosphoric acid is added, after which it is raised to about5.7.

A granular material is formed and about one hour after all of thepyrophosphoric acid solution has been added,

the agitation is discontinued, and the granular material is separatedfrom the liquid solution phase by filtration.

Using the analytical techniques as in Example II, the following analysesare obtained on samples of the crystalline material.

From these analyses the material is determined to be 002F207 8H2O.

What is claimed is:

1. A process for producing metal pyrophosphates comprising reactingpyrophosphoric acid and a metal source selected from the groupconsisting of iron, zinc, lead,

Iil)

copper, cobalt, aluminum, nickel and tin in an aqueous medium containingat least about 84 weight percent of water, at a temperature below aboutC. and at a pH from about 1.5 to about 7 to thereby form a metalpyrophosphate precipitate selected from the group consisting of iron,zinc, lead, copper, cobalt, aluminum, nickel and tin and recovering saidmetal pyrophosphate.

2. A process according to claim 1 wherein said pH is from about 2 toabout 6.

3. A process according to ciaim 1 wherein said metal source is selectedfrom the group consisting of water soluble metal hydroxides, oxides,salts of inorganic acids, salts of lower alkyl acids and mixturesthereof.

4. A process according to claim 3 wherein said metal is copper.

5. A process according to claim 3 where said metal is zinc.

6. A process according to claim 3 wherein said metal is iron.

7. A process according to claim 3 wherein said metal is nickel.

8. A process according to claim 3 wherein said metal is tin.

9. A process according to claim 3 wherein said metal is aluminum.

10. A process according to claim 3 wherein said metal is lead.

11. A process according to claim 3 wherein said metal is cobalt.

No references cited.

OSCAR R. VERTIZ, Primary Examiner.

L. A. MARSH, Assistant Examiner.

1. A PROCESS FOR PRODUCING METAL PYROPHOSPHATES COMPRISING REACTINGPYROPHOSPHORIC ACID AND A METAL SOURCE SELECTED FROM THE GROUPCONSISTING OF IRON, ZINC, LEAD, COPPER, COBALT, ALUMINUM, NICKEL AND TININ AN AQUEOUS MEDIUM CONTAINING AT LEAST ABOUT 84 WEIGHT PERCENT OFWATER, AT A TEMPERATURE BELOW ABOUT 50*C. AND AT A PH FROM ABOUT 1.5 TOABOUT 7 TO THEREBY FORM A METAL PYROPHOSPHATE PRECIPITATE SELECTED FROMTHE GROUP CONSISTING OF IRON, ZINC, LEAD, COPPER, COBALT, ALUMINUM,NICKEL AND TIN AND RECOVERING SAID METAL PYROPHOSPHATE.